<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>结构伪类选择器</title>
    <style>
        :root,body{
            height: 100%;
            background: yellow;
        }
        /* 该:root永远是html */
    </style>
</head>
<body>
   
</body>
</html>
